The mobile communication technology constantly purchases improvements in quality and quantity to satisfy up-to-date requirements. In the next-generation mobile communication system, using relays in a wireless communication network is an emerging technology to expand the system coverage, increase the overall throughput, promote the transmission efficiency, and reduce the propagation loss between a base station and a mobile station, whereby is effectively saved the transmission power of the mobile station and greatly prolonged the run time of batteries of the mobile station. If the relaying system is appropriately disposed on the boundaries of the base station coverage or the areas having a serious shadowing effect, the base station can provide uniform data rate coverage for users at different locations within the coverage. The relaying system is also used to realize transmit diversity. Owing to the transmission characteristics of the wireless channels, the destination receives signals from a direct path (source-to-destination) and a relay path (source-to-relay and relay-to-destination). The signals (carrying the same data) from different paths are combined in the destination to obtain transmit diversity, whereby the signal quality is effectively improved in the destination. Further, the relaying system also can be applied to a cooperative diversity technology to overcome multi-path fading and shadowing, especially when the source, relaying system and destination are equipped with only one antenna.
The OFDM (Orthogonal Frequency Division Multiplexing) technology is an effective modulation method to effectively increase the bandwidth efficiency and prevent inter-symbol interference (ISI) between signals. The OFDM technology has been widely applied to various wired and wireless communication systems, such as IEEE 802.16e specification, 3GPP-LTE (Long Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access).
A relaying system may work in a DF (Decode-and-Forward) mode or an AF (Amplify-and-Forward) mode. In the DF mode, the relaying system receives signals from the source, decodes and encodes the signals, and forwards the encoded signals to the destination. In the AF mode, the relaying system amplifies the signals from the source and forwards the amplified signals to the destination. Compared with the DF relaying system, the AF relaying system is free of active elements for encoding. Therefore, the AF relaying system has the advantages of low cost, easy installation and small size.
Currently, the channel estimation methods for the OFDM-based AF relaying system have the following related technologies. C. S. Patel and G. L. Stuber disclosed an LMMSE (Linear Minimum Mean Square Error) channel estimation method in a paper “Channel Estimation for Amplify and Forward Relay Based Cooperation Diversity Systems”, IEEE Trans. Wireless Commun., Vol 6, pp. 2348-2356, 2007. Fand Liu, Zhe Chen, Xin Zhang and Dacheng Yang disclosed a low rank MMSE channel estimation method in a paper “Channel Estimation for Amplify and Forward Relay in OFDM System”, International Conference on Wireless Communications, Networking and Mobile Computing, October 2008, pp. 1-4, which is based on an SVD (Singular Value Decomposition) method and exempted from inverse operation of the channel correlation matrix.
However, they are only literatures focused on the estimation of the composite channel (source-to-relay-to-destination) at the destination. So far, they have not yet been methods able to estimate individual channels of source-to-relay and relay-to-destination. The difficulty of estimating individual channels is that the signal amplified by the relaying system and transmitted to the destination no more makes the channel and noise of the destination to be presented in a Gaussian distribution. In the conventional literatures, the channel estimation method is based on MIP (Multipath Intensity Profile), which is obtained by additionally consuming system resources.
As mentioned above, the destination has to combine the signals from the direct path (source-to-destination) and the relay path (source-to-relay and relay-to-destination) to obtain the transmit diversity and effectively improve the signal quality at the destination. However, to achieve an optimal combination, the destination needs individual channels of source-to-relay and relay-to-destination to calculate the combining coefficients needed for the optimal combination at the destination.